Apparatus for recording and/or reproducing sound



Sept'.4 5, 1933. H. J. ROUND 1,925,608

I APPARATUS FOR RECORDING AND/OR REPRODUCING SOUND Filed Aug.. 26, 1929 :annu

V s n`n un are RECTIF'IER,

A U "l B AMPLIFI EE. AND

RECTI F'l EJE.

Patented Sept. 5, 1933 PATENT OFFICE PPARATUS FOR RECORDING AND/0B REPRODUCING SOUND Henry `loseph Round, Mnswell Hill, `London, England, assigner to Marconis Wireless rlelegraph Company, Limited, London, England Application August 26, i929, Serial No. 388,396, and in Great Britain September '2, 1928 3 illaims.,

En apparatus for broadcasting, making gramophone records, talking film records or the like, diiculty has been experienced owing to the fact that the differences in the volume or strength of d the sounds to be broadcast or recorded are very large.

lf, for example, there is being made on a gramophone disc a record of music, part of which is played by a large orchestra and part by solo instruments, or pianissimo passages by the or-l chestra, the solo or pianissimo passages are below the ground noise of the reproducing disc, if the loudest passages are properlyA recorded.

In order to overcome this disadvantage, various forms of control have been employed, partially to decrease the differences in the volume of sound which is broadcast or the strength of cutting of the gramophone or like record. ln some cases a musician reads the score and adjusts the volume of sound, increasing the volume of the soft passages and decreasing thefvolume of the loud pase sages. ln other cases, the variations in the cut on the record, corresponding to the volume of sound, have been kept automatically within limits.

rIhis is an advantage to the user of the record or the broadcast receiver, in which the output apparatus is of very limited power, but in cases where the output system is capable of producing a strength approximately equal to the orchestra, it is obviously a disadvantage that such a repression of the variations in the volume of the music should take place.

According to the present invention l provide 35 means for making a record of sounds so that the variations in amplitude in said record lie within limits which are` narrower than the limits which would be obtained if sounds of all amplitudes were proportionately recorded `(that is to say 4@ sounds of strong amplitude are recorded at less amplification than the amplication of sounds of weak amplitude) and I make a record of the de- 'gree of control employed. When the sounds are reproduced, the control record is utilized inversely,

to negative the control exercised on the recorded differences between the volume of the loud and soft sounds, whereby the correct variations are reproduced. si

In the case of talking lms or the like, the control record and the sound record may be recorded side by side, the former being utilized when playing off to restore the reproduced sounds to correspon'd in intensities to the sounds originally recorded.

It is already known in sound recording to make two records of the sound, (a) a record of the sound reduced to an Varbitrary and. artificial standard intensity, i. e. a record of constant arnplitude, and (b) a record of the intensity of the sound. ln the production of the (ai record the il@ instantaneous amplitudes of the recorded sound variations are controlled by the sarne energy which produces the (b) record so that as a result two sound records are produced, in one of which the amplitudeA isv constant (though it may be l5 varied from time to time), the other record being a record of the variations necessary to reduce the varying .intensity sounds to a constant amplitude. In playing oi from records so made the record of control is utilized to insert an inverse correction in an amplifier or the like in the reproducing apparatus so that sound varying in intensity in the same way as the original sound which was recorded is reproduced.

The present invention is distinguished from this z5 i I known method of recording in that the sound recorded is not reduced to an arbitrary or artificial standard intensity, i. e. it is not brought to a conn stant amplitude, the present invention consisting in recording the sound so that the variations of amplitude'in the record lie within limits which are narrower than they would be if sounds of all intensities were recorded proportionately.

Methods of effecting limitation for various purposes have been suggested, and it has been proposed to employ the actual strengthening of a sound, or a, derivative thereof, to actuate a controlling or limiting system. For example, the output from a microphone receiving sounds may be rectined and partially smoothed, so that only the larger slow variations of amplitude are left, these variations being utilized to actuate a suitable device adapted to alter the sensitivity or magniflcation of an amplifier associated with the microphone. I

With arrangements of this kind, the control will be a little late in action, and oven-modulation, due to large intensity sounds, may occur bevfore the control comes into action. To avoid this,

In some cases, for example, in so-called talking pictures it may be preferred to make two or more sound records of a sound, the said records corresponding each-to the output of one of a plurality of stereoplionically arranged microphones or the like, i. e. microphones arranged at different positions and flistening to the same sound source. nsuch cases it will generally be found that only one control record (i. e. record of the degree of control employed in recording the sounds) is necessary since the control for all the Stereophonically recorded sound records `will ordinarily be the same, but sometimes twoor even more control records will be desirable.

In arrangements in which separate sound iilms are required for "projection" at the same time, e. g. a sound nlm oi a mansinging and a separate nlm of a band playing, said iilms being recorded separately, two control records may be necessary.

lThe invention is illustrated in the accompanying schematic drawing.

Referring to Figure l, which illustrates one way of carrying out the invention as applied, for cr:-

ample, to a talking iilm, M is a microphone whosel output is fed to an amplifier A which in turn actuates means, LS, for making a sound trace SR upon a iilm. F. In this arrangement, control is eiected by hand by operating a handle H which controls the amplification of the amnlier A. The handle H also operates a variable resistance V in series with a battery B and lamp L2, the light from which is focused by a lens S2 upon the nlm F so as to give a second trace CR which, as will be seen, is a record of the degree of control employed.

In the modiiication 4shown in Figure 2. the control and recording of the degree of control is eiiected automatically. In the arrangement illustrated, the amplification oi the sound amplier A is controlled by impressing 4a varying bias potential upon the grid of an auxiliary valve X, associated with theampliiier A in some such way as is illustrated in Figure 2, in which the valve X forms with the resistance R an alternating current potentiometer controlled in accordance with the output from the microphone M. This control is eected by means oi a separate ampliiier B. which incorporates. amplifier valves, a rectider, and some i'orm of smoothing circuit adapted u' `lin'iinate all but slow variations. As will be seen, the amplier B has its input side fed from the microphone M, While its rectified and smoothed output is utilized to apply controlling grid bias to the valve X. In this way the amplication of the amplifier A is directly controlled by the strength of sound. The control box B Yet containing the rectifier also feeds energy to the lamp L2 which makes a record of control, as in Figure l.

Figure 3 shows a further modincation, didering from that shown in Figure 2 only in that a delay device DC is interposed between the microphone M and the amplier A. This delay device may comprise a circuit consisting o an artidcial, and as nearly as possible distortionless, line having a time delay of, say one tenth or one iifth of a sound such as series impedances DCL and parallel capacities DCC interposed between the microphone M vand the ampliiier A. Or the delay device DC may comprise an arrangement in which the soundl is delayed by its eiiect being caused to pass through a mechanical line of solid liquid or gas, ther delay in this case being occasioned by the length of time taken by the sound vibrations to traverse the line or the delay device aeeaece DC may be constituted by a telegraphone, the time delay being in this case 'ls-Ji to the timey taken by the magnetized wire or the like to pass from the point at which the magnetism oi a movingendless wire or the like is varied by sound derivates to the point at which said variations are transformed back into electrical variations. litwill be seen that with such arrangements in accordance with this invention the control will be edected a little early, and in consequence the desired limitation will be satisfactorily obtained. f

Preferably the control is e slightly sluggish in recovery, so as to overlap the sound slightly at the end of the time necessary for said control. Y

Figure i illustrates an arrangement for playing on from a record de, 'or example, by the apparatus shown in Figure 2. ln this L3 is a source ofvlight which is focused by a lens system S3 upon the trace SR. Light modulated in accordance with said trace falls upon a photoelectric cell Pl whose output is amplified by means oi an ampliiier n' feeding' a loud speaker LS to reproduce the sound. A sound lamp lens and im photo-electric cell ksystem L4 Sd P2 pick up from the control" trace CR, the output from the cell P?. being fed to anampliner CA which serves to control the amplidcation of the plider A in a manner analogous to that in which control box B controls the amplider A (e 2), so that in playing od the control orig .1 exercised is negatived and the variations in the volume of sound are correctly reproduced.

The time delay means illustrated in Figs. 2 and 3 may be readily applied to the circuit arrangement shown in Fig, l. The time delay feature of my invention is applicable to all of the embodiments illustrated. im

Where two or more sound records are made of a single sound by means of sterecphonically arranged microphones, as hereinbeiore described,

idd

'the record or records of control may be made by the side oi the sound records on a single lm, if desired. e said sound records may be constituted in any well known way-for example, by photographically recording the movement oi' a spot or spots of lightand are made' by recording the electric variations (or variations corresponding thereto) obtained from a pair of Stereophonically arranged microphones which are subjected to the influence of the sound to be recorded, one record lcorresponding to the output from one microphone and the `other to that from the other. l

In reproducing (for talking pictures) from stereophonic nlms as above described, the two records are-employed each for controlling one of two loud speaking devices arranged one on either side of the screen on which the picturesl l are projected.

.In some cases the nlm or may comprise three or even four sound records, so as to represent up and down, as well as sideways motion, to an audience, the said records being utilized to M@ operate each one of a plurality of appropriately' arranged loud speakers.

it will be understood that although the invention has for the'most part been described with reference to sound records made upon it is M5 equally l applicable to other kinds of sound recordstor example, gramophone records.

What I claim isil. A sound recording system comprising a lm adapted to receive a pair of light records, a pair l5 of recording devices adjacent said lm for impressing variable records thereon, a microphone circuit, a pair of amplifying channels having their input circuits connected with said microphone circuit, the output circuit of one of saidv channels being connected with one of said recording devices and the output circuitoi the other or said 4channels being connected with the other of said recording devices, one of said channels including a multipiestage ampiier having coupling means interconnecting said stages, the other of said channels including a rectifier and an electron tube amplifier', an auxiliary electron tube having its input circuit connected with the coupling means in said iirst mentioned ampliner and its output circuit connected with said last inentioned electron tube amplifier for variably controlling the operating characteristics thereof and correspondingly controlling the intensity of the associated recording device for impressing a variable control record on said lm in addition to the recording or a variable sound record under control of the recording device connected with the output of said iirst mentioned multiple stage amplifier.

2. A sound recording system comprising a nim adapted to receive a pair of light records, e. pair of recording devices adjacent said nlm for irn pressing variable records thereon, a microphone circuit, a pair oi amplifying channels having their input circuits connected with said microother o said channels including a rectifier and an electron tube amplifier, an auxiliary electron tube having its input circuit connected with the coupling means in said rst mentioned amplifier and its output circuit connected with said last mentioned electron tube amplifier or variably.

controlling the operating characteristics thereof and correspondingly controlling the intensity oi the associated recording device for impressing a variable control record on said film in addition to the recording of a variable sound record under controloi the 4recording device connected with the output of said nrst mentioned multiple stage amplier, and means for actuatingsaid second mentioned amplifier in advance of said first mentioned amplifier for displacing the positions of the variable sound record and the variable control record on said film.

3. A sound recording system comprising a iilm adapted to receive a pair of light records, a

oi? said channels including a reciiier and an elec-s tron tube amplifier, an auxiliary electron tube having its input circuit connected with the coupling means in said iirst mentioned amplier and its output circuit connected with said lastinentioned electron tube ampliiier for variably controlling the operating characteristics thereof and correspondinghr controlling the intensity of the associated recording device for impressing a variable control record on said lrn in addition to the recording of the variable sound record under control oi the recording device connected with the output oi said rst mentioned multiple stage amplifier, and means comprising a multiplicity of series connected impedances and a plurality oi shunt connected impedances disposed between said microphone circuit and said ilrst mentioned amplifier for delaying the operation oI the recording device in the output or said nrst inenmit tioned amplifier with respect to the operation oi the recording device in the output of said second mentioned ampliiier for displacing the control record from the sound record on the iilrn.

` i-mNRY JOSEPH RUUND.

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